1. Field of the Invention
This invention relates generally to an integrated cell voltage unit for monitoring a fuel cell stack and, more particularly, to an integrated cell voltage unit for monitoring a fuel cell stack that includes snap fit electrical connections to easily connect the bipolar plates of the fuel cell stack to the unit.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode charge gas that includes oxygen, and is typically a flow of forced air from a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. Also, the fuel cell stack receives an anode hydrogen gas.
Each fuel cell in the fuel cell stack includes opposing bipolar plates having flow channels through which the anode gas, the cathode gas and a cooling fluid flow. A cell membrane is positioned between the bipolar plates in each fuel cell, and receives the cathode gas and the anode gas to generate the electricity in the manner discussed above. The bipolar plates are conductive members, such as stainless steel, that are coupled in series and collect the electrical current generated by the fuel cell stack to be output therefrom. In a typical fuel cell stack for an automotive application, there are about 200 fuel cells, and thus, about 200 bipolar plates.
It is necessary to monitor the electrical potential of each bipolar plate during operation of the fuel cell stack to ensure that each fuel cell in the stack is operating properly. If one of the fuel cells in the stack is not generating the proper amount of current, the entire stack could be damaged. Therefore, each bipolar plate is electrically coupled to a cell voltage unit (CVU) that monitors the voltage of each cell and the overall output power of the fuel cell stack.
FIG. 1 is a top plan view of a known fuel cell system 10 including a fuel cell stack 12 mounted in a fuel cell module housing 14. The fuel cell stack 12 includes a series of bipolar plates, as discussed above, each including an electrical tab 16 to which an electrical wire 20 is electrically coupled. The end of the wire 20 includes an electrical connector (not shown) that fits onto the tab 16 in an electrical friction-fit engagement. The system 10 further includes a plurality of CVUs 22 electrically coupled in series, and positioned within a housing 24. The CVUs 22 monitor the voltage output of each individual cell within the stack 12 and the total output voltage of the stack. To accomplish this, each of the wires 20 is coupled to a certain one of the CVUs 22. For the fuel cell system 10, six of the adjacent wires 20 are coupled to a common connector plug or wire harness 26 that is then coupled to the appropriate CVU 22.
In the known fuel cell systems, each electrical connection to the bipolar plates and the CVUs 22 are performed manually. Because there are typically a few hundred cells in the fuel cell stack 12, manually coupling the wire harnesses 26 to the CVU 22 and the wires 20 to the bipolar plate tabs is extremely labor intensive. Other disadvantages are also present in this type of assembly process. For example, it is possible to interchange the wires 20 during assembly so that they are not connected to the proper bipolar plate. Further, the wires 20 and harnesses 26 require a significant amount of space. Also, the CVUs 22 require their own housing separate from the fuel cell module housing 14. Furthermore, the assembly process is not designed for manufacturing and assembly.